1. Field of the Invention
The present invention relates to a spatial optical transmission apparatus for transmitting a light beam in space, and in particular to a spatial optical transmission apparatus and a spatial optical transmission method for performing a spatial optical transmission by utilizing the coherency of a laser beam.
2. Description of the Related Art
Conventionally, by means of optical hetero-dyne wavedetection, a spatial optical transmitting method utilizing the coherency of a laser beam (hereinafter, referred to as an optical heterodyne detection transmission method) has provided a transmission at a high S/N ratio (signal to noise ratio) compared with that provided by a spatial optical transmitting method based on light intensity modulation and direct wavedetection (hereinafter, referred to as an optical direct detection transmission method). In addition, the optical heterodyne transmission detection method is characteristically less affected by incoherent light (background light) existing in the natural world. Thus, when using this method, a transmission at a transmission speed of e.g., 10 Mbps (10 megabits per second) can be performed with a minimum receiving light power equal to or less than about one-eighth of that in the case of the optical direct detection transmission method. This assures long-distance and high-speed transmissions at approximately 10 Mbps, which is a transmission speed required for transmission of a digital moving image or the like but difficult to realize by the optical direct detection transmission method.
Referring to FIG. 22, a line P.sub.0d shows the relationship between the transmission speed and the transmission distance which are obtainable at a given receiving light power by an optical direct detection transmission method. A line P.sub.0c shows the relationship between the transmission speed and the transmission distance which are obtainable at a given receiving. light power (equal to the given receiving light power in the optical direct detection transmission method) by a coherent detection method. As is seen from FIG. 22, at a transmission speed of 10 Mbps, the transmission distance can be extended to a point where the receiving light power is about one-eighth as large as that obtained in the optical direct detection transmission method.
In the above-mentioned manner, when a receiving light power is constant, the relationship between the transmission distance and the transmission speed is defined by the product of the transmission distance and the transmission speed. As the receiving light power. becomes larger, the transmission distance and the transmission speed can be increased.
One configuration of an optical heterodyne detection transmission method will be described. A transmission signal beam source is optically frequency-modulated by two kinds of frequencies (f.sub.1, f.sub.2) in accordance with a signal to be sent (hereinafter, referred to as a transmission signal) like (0, 1) to generate the signal beam into the air. The output signal beam is mixed with a locally oscillated beam (frequency f.sub.1) by a signal demodulating device, to be detected by a detector. A signal having only a DC component which corresponds to data 0 and a beat signal of a frequency (f.sub.2 -f.sub.1) which corresponds to data 1 are obtained from an output signal of the detector. The data can be demodulated from the DC component and the beat signal.
The conventional optical heterodyne detection transmission method has a problem that, in order to demodulate the signal by a wavedetector in a signal demodulating device, an angular difference between the wavefront of a signal beam output from a signal transmitting device located at a spatially arbitrary position and that of a locally oscillated beam in the signal demodulating device should be adjusted to be within 0.01.degree..
As a spatial optical transmission apparatus described above, a configuration as shown in FIG. 23 has been proposed by Japanese Patent Application No. 4-349403. In the proposed apparatus, a signal transmitting device located at an arbitrary position and a signal demodulating device 2 are apart from each other with a sufficient distance therebetween. Furthermore, the signal demodulating device 2 has an aperture for limiting an area through which a beam enters. Accordingly, on a light receiving face of a wavedetector 7, a transmission signal beam becomes a plane wave. A locally oscillated beam from a locally oscillated beam source 4 provided in the signal demodulating device 2 is collimated by a lens 5. The collimated beam is diffused into spherical waves by such a means as a microlens array 6 to be mixed with the transmission signal beam by an optical multiplexer 3. Herein, the locally oscillated beam has plane wave components in any of the unit areas on the light receiving face of the wavedetector 7. Accordingly, among the wavefronts of the spherical waves exists a wavefront which can be aligned (synchronized) with that of the transmission signal beam propagated from an arbitrary direction, and thus, the transmission signal can be detected.
As another wavefront alignment technique, there has been known a spatial optical transmission between space satellites. As shown in FIG. 24, a beacon light beam 12, which is intensity-modulated as a countermeasure against background light, is transmitted from a beacon light source 11 of the satellite on the signal transmitting side. On the other hand, a signal transmission light source 15, driven by a laser power source 13, generates a signal beam modulated by a modulating device 14 in accordance with a transmission signal. The signal beam is collimated by a lens 16, and output in the space via a precisely directing control mirror 17 and a roughly directing control mirror 18.
In a satellite on the signal receiving side, an output signal of a two-dimensional CCD (charge coupling device) 19 provided for capturing the signal beam is input to a capturing/tracking control circuit 20. A mirror drive device 21 is driven by this capturing/tracking control circuit 20 so as to control the roughly directing control mirror 18, whereby an incident beam is roughly adjusted and input to a four quadrant detector 22. Thereafter, the precisely directing mirror 17 is controlled by the capturing/tracking control circuit 20 based on an output from the four quadrant detector 22, for performing tracking of the signal beam. Then, the received signal beam is received by a signal transmission light receiving element 23. Thereafter, the signal is demodulated in a demodulation circuit 25 via an amplifier 24.
At the time of receiving the signal beam, a superior S/N ratio is obtained by making the spot size of the locally oscillated beam small. However, it is difficult to perform wavefront alignment for a beam with a small spot. For this reason, the wavefront aligning method making a locally oscillated beam diffuse has been used. In this case, however, there occurs a problem that the intensity of the locally oscillated beam is weakened by diffusion.
With reference to FIGS. 25A and 25B, an exemplary configuration of a system using this method will be described. A signal demodulating device 32 is located at a point apart from a portion just below a signal transmitting device 31 by a radius L2 (e.g., 20 m), with a distance L1 (e.g., 5 m) in the vertical direction between the plane on which the signal transmitting device 31 is located and the plane on which the signal demodulating device 32 is located. The size of a signal beam receiving aperture 33 of the signal demodulating device 32 is 0.5 cm.times.0.5 cm. An optical multiplexer 35 for mixing the locally oscillated beam from a locally oscillated beam source 34 and the signal beam is apart from a photodetector 36 by a distance L3 (e.g., 10 cm). In such a configuration, the intensity of the signal beam is 1/100 as compared with the intensity obtained by a structure which does not employ diffusion of the locally oscillated beam. This results in reduction of the intensity of the locally oscillated beam, which intensity would be sufficiently obtained by conventional techniques. Thus, the above-mentioned method has a problem that a high-speed transmission, which is an advantage of the optical heterodyne detection transmission, is not realized.
On the other hand, in the method used for an inter-satellite spatial optical transmission, the beacon light source 11 and the signal transmission light source 15 are provided on the transmitting side. Thus, since two light sources are necessary for this method, it can not be applied for a civilian use, which requires low cost, compactness, and low energy consumption.
Another technique is known where the wavefront of a signal beam and that of a locally oscillated beam are aligned by using a mechanical means for causing the locally oscillated beam to scan space. For the wavefront alignment, it is necessary to scan an entire space within an angular range of 0.degree. to 15.degree. in all directions in steps of 0.01.degree.. As a result, a long time period is required as a response time of the wavefront synchronization time. Moreover, such a scanning has to be conducted each time the signal demodulating device is operated. Accordingly, this technique is not practical.